The traditional method of achieving automotive heater/air conditioning blower speed control is by use of an open coil resistor assembly consisting of one or more individual coil elements, usually connected electrically in series. Operation of the blower switch located on the vehicle instrument panel connects the blower motor to none, one, two or more of the resistance elements to progressively decrease the speed of the motor from its highest speed to lower ones. An advantage of the design is that the individual resistance values of the elements may readily be changed to optimize performance of an individual vehicle system design. The resistor assembly is usually located downstream of the motor and blower in the climate control air ducts built into the vehicle whereby the moving airstream cools the elements during normal operation. During a fault condition, such as failure of the blower motor shaft to rotate (locked rotor), open coil resistors may reach unacceptably high temperatures. A thermal fuse located above the resistance elements is often employed to limit the temperature rise during a fault condition by opening the resistor and motor circuit in response to an increase in convected and radiated heat from a resistance element. In other applications, the resistor assembly without a thermal fuse is located in an area where high temperatures will not adversely affect the surroundings.
Some other resistor products use flat plates, relying on resistive ink elements screen printed on either a ceramic or an enameled metal base and utilizing melting solder connections between the resistive elements to limit temperature rise during fault conditions.